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Shared Memory

Overview

There are two types of shared memory objects in Linux: System V and POSIX. The containers in a pod share the IPC namespace of the pod infrastructure container and so are able to share the System V shared memory objects. This document describes how they can also share POSIX shared memory objects.

POSIX Shared Memory

POSIX shared memory requires that a tmpfs be mounted at /dev/shm. The containers in a pod do not share their mount namespaces so we use volumes to provide the same /dev/shm into each container in a pod. The following example shows how to set up POSIX shared memory between two containers.

shared-memory.yaml
---
apiVersion: v1
id: hello-openshift
kind: Pod
metadata:
  name: hello-openshift
  labels:
    name: hello-openshift
spec:
  volumes:                          (1)
    - name: dshm
      emptyDir:
        medium: Memory
  containers:
    - image: kubernetes/pause
      name: hello-container1
      ports:
        - containerPort: 8080
          hostPort: 6061
      volumeMounts:                 (2)
        - mountPath: /dev/shm
          name: dshm
    - image: kubernetes/pause
      name: hello-container2
      ports:
        - containerPort: 8081
          hostPort: 6062
      volumeMounts:                 (3)
        - mountPath: /dev/shm
          name: dshm
1 specifies the tmpfs volume dshm.
2 enables POSIX shared memory for hello-container1 via dshm.
3 enables POSIX shared memory for hello-container2 via dshm.

Create the pod using the shared-memory.yaml file:

$ oc create -f shared-memory.yaml